The present invention generally relates to the chemical treatment of a fibrous material, and more particularly relates to the movement of chemicals through a fibrous material using low-headspace dielectric heating.
Uniform chemical treatment of wood is a challenge in the paper making industry. One purpose for chemical treatment of wood is to soften wood chips so that lignin can more easily be separated from fiber for use in papermaking. If chemicals do not integrate throughout the wood, that portion of the wood that has not been treated is usually regarded as waste or requires further processing due to the inability to separate the fibers and lignin.
In chemical pulping, the wood is disintegrated through cooking in the presence of a catalyst. The ingress of the catalyst into the wood matrix is frequently rate limiting, and the speed and uniformity of pulping may be impacted due to the catalyst""s poor penetration into the wood matrix. Chemical additives that improve penetration of the catalyst into wood are known in the industry as chip penetrants and are described in U.S. Pat. No. 4,906,331 to Blackstone et al., which is incorporated herein by reference. These chemical additives (referred to herein as simply xe2x80x9cadditivesxe2x80x9d) and catalysts can be expensive.
In the prior art, the use of microwave irradiation for enhancing diffusion in polymeric materials is described. Specifically, one prior art method describes how the diffusion of the gas, ethylene oxide, in polyvinyl chloride is enhanced by microwave irradiation. Gibson et al., xe2x80x9cMicrowave Enhanced Diffusion In Polymeric Materials,xe2x80x9d International Microwave Power Institute 23, 17-18 (1988). Gibson et al. compared diffusion rates of the ethylene oxide in an oven to those in a cycled microwave field and found that the activation energy for diffusion was lowered considerably during microwave irradiation. Hence, the rate of diffusion in the microwave field was enhanced to an extent that exceeded the rate increase from heating. n important point to note in this prior art method is that the diffusing component, ethylene oxide, is a gas, which is naturally predisposed to diffusion into a solid.
The use of irradiated microwave energy was again applied in the prior art for the purpose of moving terpenes out of green softwood as described in U.S. Pat. No. 6,029,368 to Banerjee et al., which is incorporated in its entirety herein by reference. Banerjee et al. describes a process by which softwood flakes for the manufacture of oriented strand board, or softwood dimensional lumber are irradiated by thermal, radio frequency, or microwave energy under low-headspace conditions, such that water loss from the wood due to drying is limited to less than 10 percent by weight. A substantial fraction of the turpentine contained within the wood is removed from within the wood during the period of irradiation. However, Baneree et al. does not address the issue of enhancing the movement of chemicals into the wood to improve pulping or preservative treatment.
Therefore, there is a need for a system and method for improving the distribution of chemicals through a fibrous material. There is also a need for a system and method for preserving the moisture content of a fibrous material so as to facilitate the diffusion of chemicals into the fibrous material. There is yet another need for a system and method for the chemical treatment of a fibrous material that does not require the addition of additives. There is yet another need for a system and method that diffuses the chemicals into a fibrous material uniformly and rapidly.
The present invention solves the above-described needs by providing a system and method for distributing chemicals in a fibrous material, such as wood. Chemicals are applied to the fibrous material. Next, the fibrous material is subjected to thermal energy and/or dielectric heating, such as microwave or radio frequency energy, under conditions which significantly reduce water evaporation within the fibrous material. As a result, the chemicals are able to distribute evenly and quickly throughout the fibrous material.
In one aspect, an improved method for diffusing a chemical into a fibrous material comprises applying a chemical to a fibrous material. The fibrous material is then placed in a low-headspace environment and irradiated with dielectric heat until the chemical is diffused into the fibrous material.
The fibrous material may be selected from a group consisting of softwood, hardwood, bamboo, papyrus, paper, and straw. The dielectric heat may be either microwave energy or radio frequency energy. The low-headspace environment is designed to inhibit evaporation of water from the fibrous material, wherein it is preferable that the low-headspace environment allows at most about twenty percent evaporation of water from the fibrous material. For example, the fibrous material may be covered with an object, such as plastic, that inhibits evaporation of water from the fibrous material. Alternatively, the fibrous material may be placed in a low-headspace dielectric heater, or a dielectric heater can be filled with the fibrous material such that the amount of space remaining within the dielectric heater inhibits evaporation of water from the fibrous material.
The chemical is applied to at least one surface of the fibrous material. However, the fibrous material may be immersed in the chemical. The chemical used in the present invention is a catalyst used for pulping the fibrous material, such as a sulfur-derived compound like sodium sulfide. The chemical may also be white liquor or an anthraquinone. The chemical may also be a preservative used for preserving the fibrous material, such as borate and chromium compounds.
In another aspect, an improved method for diffusing a chemical into a fibrous material in accordance with the present invention comprises applying a chemical to a fibrous material, placing the fibrous material in a low-headspace environment, irradiating the fibrous material with thermal heat, and then irradiating the fibrous material with dielectric heat until the chemical is diffused into the fibrous material.
In yet another aspect, a system for diffusing a chemical into a fibrous material consistent with the present invention comprises a chemical treater for applying a chemical to a fibrous material and a dielectric heater for irradiating the fibrous material with dielectric heat until the chemical is diffused into the fibrous material.
The chemical treater can be a tank containing the chemical, wherein the chemical is applied to the fibrous material by immersing the fibrous material in the tank. The chemical treater can also be a sprayer for spraying the chemical onto the fibrous material. The dielectric heater preferably has a volume sized to provide a low-headspace environment that inhibits evaporation of water within the fibrous material. Preferably, the low-headspace environment allows at most about twenty percent evaporation of water from the fibrous material. In addition, the dielectric heat can be microwave energy ranging from about 300 GHz to about 3 KHz or the dielectric heat can be radio frequency energy ranging from about 300 MHz to about 3 KHz.
In this aspect, the system may further comprise a conveyor for transporting the fibrous material through the dielectric heater, where the dielectric heater has an entry location for allowing the conveyor to transport the fibrous material into the dielectric heater and an exit location for allowing the conveyor to transport the fibrous material out of the dielectric heater.
These and other objects, features, and advantages of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.